I think their explanation is great. The shader is run on all the pixels within the quad and your shader code needs to figure out if the pixel is within the shape you want to draw or not. Compared to just drawing it pixel by pixel if you do it by pen or on the CPU.

For a red line between A and B:

CPU/pen: for each pixel between A and B: draw red

GPU/shader: for all pixels: draw red if it's on the intersection between A and B

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dahart 3 hours ago | parent [-]

Figuring out if a pixel is within a shape, or is on the A-B intersection line, is part of the rasterizing step, not the shading. At least in the parent’s analogy. There are quite a few different ways to draw a red line between two points.

Also using CPU and GPU here isn’t correct. There is no difference in the way CPUs and GPUs draw things unless you choose different drawing algorithms.

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Kiro 2 hours ago | parent [-]

While (I presume) technically correct I don't think your clarifications are helpful for someone trying to understand shaders. The only thing that made me understand (fragment) shaders was something similar to the parent's explanation. Do you have anything better?

It's not about the correct way to draw a square or a line but using something simple to illustrate the difference. How would you make a shader drawing a 10x10 pixels red square on shadertoy?

	▲	dahart an hour ago \| parent [-]
		You’re asking a strange question that doesn’t get at why shaders exist. If you actually want to understand them, you must understand the bigger picture of how they fit into the pipeline, and what they are designed to do. You can do line drawing on a CPU or GPU, and you don’t need to reach for shaders to do that. Shaders are not necessarily the right tool for that job, which is why comparing shaders to pen drawing makes it seems like someone is confused about what they want. ShaderToy is fun and awesome, but it’s fundamentally a confusing abuse of what shaders were intended for. When you ask how to make a 10x10 pixel square, you’re asking how to make a procedural texture with a red square, you’re imposing a non-standard method of rendering on your question, and failing to talk about the way shaders work normally. To draw a red square the easy way, you render a quad (pair of triangles) and you assign a shader that returns red unconditionally. You tell the rasterizer the pixel coordinate corners of your square, and it figures out which pixels are in between the corners, before the shader is ever called.